The post-genome era is stimulating a strong demand for high-throughput assays uncovering the individual behavior of single cells in populations. The pharmaceutical industry also requires high-throughput cell-based drug screening assays, to reduce the cost and ethical issues of animal testing, better assess effects and toxicity on human cells, and minimize human and economical risks of clinical trials. Powerful single cell screening methods are also highly wanted in oncology, due to the genetic variability of tumor cells. The same type of demand also concerns viruses, for which high throughput screening is a strong need. Viruses mutate very fast, and their pathogenic power is directly dependent on this rate of mutations. It is thus of paramount importance to be able to study and anticipate virus evolution and mutation, e.g. to anticipate and prepare adequate responses to the risk of pandemics.
In material sciences, too, there is a high demand for methods able to encapsulate colloidal objects such as latex particles, metal particles, magnetic material, or various dielectric materials in well-controlled envelopes of another material, and to sort different colloidal objects. For instance U.S. Pat. No. 6,558,665 to Mrksich discloses a method for encapsulating particles, involving two immiscible liquids of different densities provided in a container as upper and lower liquids, and sucking liquid from said container through a tube making a sprout of the lower, high density liquid in the stream of upper, low density liquid. This method, however, imposes that the material to encapsulate the particles be less viscous and denser than the driving liquid. It also requires large volume of liquids, and it is thus not well adapted to the manipulation of rare or expensive products. Finally, to change the thickness of the surrounding layer, i.e. the size of the final particles, it is necessary to change the viscosity of the upper fluid, which makes the device unpractical for many applications.
Microfabricated and micro fluidic devices raise the hope of a dramatic breakthrough for the preparation, manipulation and sorting of microscopic and nanoscopic objects. For instance, WO 2004/002627 to Anna et al., US patent application 20050221339 to Griffiths et al, or WO 2006/040551 to Griffiths, disclose different ways for compartmentalizing a fluid containing species into microdroplets carried in an immiscible fluid, and performing different screening regarding these species inside said microdroplets, using microfluidic control.
Microfluidics is particularly interesting for cell assays, as reviewed e.g. in El-Ali, J., Sorger, P. K. & Jensen, K. F. Cells on chips. Nature 442, 403-411 (2006), incorporated herein by reference. Encapsulated cells can be precisely manipulated using optical traps, as disclosed e.g. in He, M. et al. Anal. Chem. 77, 1539-1544 (2005), incorporated herein by reference, but this requires complex and slow technologies. In contrast, cell encapsulation in drops using flow focusing geometries offer potential for very high throughput. So far, however, attempts in this direction, as disclosed e.g. in Tan, Y.-C., Hettiarachchi, K., Siu, M., Pan, Y.-R. & Lee, A. P. J. Am. Chem. Soc. 128, 5656-5658 (2006), or in US patent application 20060163385 to Link rely on a high dilution of cells in order to avoid the encapsulation of several cells in the same drop, and require a delicate and complex sorting of “positive” droplets by flow-cytometry technologies.